entrySet用法以及遍历map的用法

Bertha 。 2022-03-31 07:49 351阅读 0赞

keySet是键的集合，Set里面的类型即key的类型  
entrySet是 键-值 对的集合，Set里面的类型是Map.Entry

1.keySet()
    Map map=new HashMap();
    Iterator it=map.keySet().iterator();
        
    Object key;
    Object value;
    while(it.hasNext()){   
    key=it.next();    
    value=map.get(key);    
    System.out.println(key+":"+value);    
    }    
    2.entrySet()
    Map map=new HashMap();
    Iterator it=map.entrySet().iterator();
    
    Object key;
    Object value;   
    while(it.hasNext()){
    Map.Entry entry = (Map.Entry)it.next();
    key=entry.getKey();
    value=entry.getValue();
    System.out.println(key+"="+value);
    }
    
    public static void main(String[] args) {
    
      Map<String, String> map = new HashMap<String, String>();
      map.put("1", "value1");
      map.put("2", "value2");
      map.put("3", "value3");
      
      //第一种：普遍使用，二次取值
      System.out.println("通过Map.keySet遍历key和value：");
      for (String key : map.keySet()) {
       System.out.println("key= "+ key + " and value= " + map.get(key));
      }
      
      //第二种
      System.out.println("通过Map.entrySet使用iterator遍历key和value：");
      Iterator<Map.Entry<String, String>> it = map.entrySet().iterator();
      while (it.hasNext()) {
       Map.Entry<String, String> entry = it.next();
       System.out.println("key= " + entry.getKey() + " and value= " + entry.getValue());
      }
      
      //第三种：推荐，尤其是容量大时
      System.out.println("通过Map.entrySet遍历key和value");
      for (Map.Entry<String, String> entry : map.entrySet()) {
       System.out.println("key= " + entry.getKey() + " and value= " + entry.getValue());
      }
    
      //第四种
      System.out.println("通过Map.values()遍历所有的value，但不能遍历key");
      for (String v : map.values()) {
       System.out.println("value= " + v);
      }
     }